The present invention relates to the manufacture of electrically conductive composite materials comprising a conductive polymer such as polyaniline, in an insulating substrate.
It has particular application to the manufacture of porous membranes based on polymers and other insulating materials, rendered conductive by the conductive polymer.
Such materials can be used as electrodes, as gas sensors, as biological microsensors, or as filtration material for inflammable liquids.
Various methods are known which enable composite materials comprising a conductive polymer to be prepared.
Thus, the document Synthetic Metals, 60, 1993, pages 27-30 [1] describes the preparation of a composite polyaniline-poly (bisphenol-A carbonate) membrane used for the detection of ammonia. This composite membrane is obtained by electropolymerization of aniline on an electrode coated with polycarbonate. It contains about 50% by weight of polyaniline and has a conductivity of 10xe2x88x922 S.cmxe2x88x921.
The document Anal. Chem., 1999, 71, pages 2231-2236 [21] describes sensors constituted by an isoporous membrane of polycarbonate coated with gold, in the pores of which a polyaniline is caused to grow by electropolymerization. An enzyme is then immobilized on the polyaniline by an electrochemical method.
The document Anal. Chem., 1998, 70, pages 3946-3951 [3] likewise describes biosensors comprising a composite electrode based on polyaniline and on Nafion(copyright) perfluorinated ionomer, which is obtained by deposition of the polyaniline by electropolymerization on a vitreous carbon electrode coated with Nafion(copyright).
The document Synthetics Metals, 84, 1997, pages 107-108 [4] describes the preparation of a composite material based on porous glass and polyaniline obtained by the polymerization of aniline by chemical oxidation in situ in the pores of the porous glass.
The document Chem. Mater., 1994, 6, pages 1109-1112 [5] likewise describes a porous material in the pores of which polyaniline is formed by chemical polymerization in situ.
The methods described hereinabove for obtaining composites comprising a conductive polyaniline deposit polyaniline by also making use of electropolymerization or by chemical polymerization of aniline, and this has certain disadvantages.
The methods based on electropolymerization in fact make it necessary to first coat the insulating membrane with an electrically conductive material to permit the growth of polyaniline by electropolymerization. Such methods are furthermore not well suited to the preparation of membranes with large surfaces, since the electric field can be very inhomogeneous in an electrolytic cell of large dimensions, leading to the inhomogeneous deposition of conductive polymer. Furthermore, the electropolymerization reactions are very slow. Moreover, it is necessary to subject the membrane obtained by electropolymerization to a subsequent washing for eliminating the residues of salt and of electrolysis solvent, which could have a negative effect on the behavior of the membrane. Lastly, it should be noted that implementation of the process is lengthy.
In the methods using polymerization in situ by chemical means in the pores of the membrane, the process is difficult to control and the deposition of the conductive polymer can be inhomogeneous due to several factors which locally influence the chemical potential. Likewise, the product obtained has to be carefully washed to eliminate the secondary products of the reaction which would have a deleterious effect on the properties of the membrane, and the implementation of this method is likewise lengthy.
Another path for obtaining a film of composite material based on insulating polymer and conductive polymer, described in WO-A-98/05040 [6], consists of starting from a solution of conductive polymer and of insulating polymer in an appropriate solvent and forming a film by casting the solution and by evaporating the solvent. However, such a method is not suitable for obtaining conductive porous membranes.
The present invention has specifically as its object a method for preparing an electrically conductive composite material comprising a porous substrate made conductive by the deposition of a conductive polymer within the pores of the substrate.
According to the invention, the method for preparing an electrically conductive composite material comprising an insulating porous substrate and a conductive polymer disposed in the pores of the insulating substrate is characterized in that it consists in performing at least one cycle of deposition of the conductive polymer comprising the following steps:
(a) putting the porous substrate in contact with a solution of conductive polymer in a volatile organic solvent, chemically inert with respect to the porous substrate, and
(b) eliminating the volatile organic solvent by evaporation for forming a deposit of conductive polymer in the pores of the porous substrate.
Generally, several successive cycles of deposition are performed, for example, three cycles of deposition, for obtaining a sufficient quantity of conductive polymer, not only in the pores but likewise on the external surface of the substrate.
The method of the invention is very advantageous, since it enables the deposition of conductive polymer to be effected in a single step, much easier and more rapid to implement than the steps necessary to perform a deposition by electropolymerization or by chemical polymerization in situ, and furthermore omitting the steps of washing.
According to the invention, the important characteristic is the choice of the volatile organic solvent used for forming the solution for deposition of conductive polymer within the pores of the porous substrate.
The solvent used should be chemically inert with respect to the porous substrate, that is, it should neither dissolve nor damage this substrate, and should ensure good dissolution of the conductive polymer.
In the case of polyaniline, it is known, for example, that this can be solubilized in solvents such as meta-cresol, as described in the document WO-A-99/07766 [7] as well as in the document [6] cited previously. But such solutions cannot be used for introducing polyaniline into a porous polymer substrate, because they likewise dissolve numerous insulating polymers.
In the document Synthetics Metals, 48, 1992, pages 91-97 [8], it is mentioned that polyanilines can be dissolved in certain solvents such as N-methyl pyrrolidone (NMP), certain amines, concentrated sulfuric acid or other strong acids, but in the case of NMP, it is necessary to then dope the polyaniline, which has become insulating. Furthermore, it is stated in this document that a polyaniline of high molecular weight cannot be doped in the conductive form, then dissolved in the conductive form in the usual, polar or weakly polar, organic solvents. According to this document, particular doping agents are used in order to place the polyaniline in solution in solvents such as meta-cresol, chloroform, and xylene.
According to the invention, other solvents are chosen, permitting:
(a) keeping the conductive polymer in the conductive form,
(b) facilitating its penetration into the pores of the porous substrate, and
(c) carrying out a uniform deposition of the conductive polymer.
With this object, solvents are chosen which are capable of dissolving a sufficient quantity of conductive polymer to form a solution containing, for example, 1-10 g/l of conductive polymer, and having an appropriate viscosity, for wetting the surface of the substrate. Moreover, an amphiphilic organic solvent is preferably chosen for obtaining a uniform deposit of conductive polymer on the hydrophilic and hydrophobic surfaces of the substrate.
By way of example of organic solvents which can be used, there can be mentioned acetic acid, the halogenated derivatives of acetic acid such as trifluoroacetic acid, and the fluorinated alcohols such as hexafluoroisopropanol.
According to the invention, the conductive polymer can be chosen among polyanilines, polypyrroles, polythiophenes and derivatives thereof.
According to the invention, a polyaniline is advantageously used, preferably of high molecular weight, and more preferably in the form of emeraldine base. Polyanilines of this type can be obtained by the methods described in the document [7] and the document Synthetics Metals, 95, 1998, pages 29-45 [9].
In the case in which the conductive polymer is a polyaniline, the solution used is advantageously a solution of polyaniline and of protonating agent in an amphiphilic volatile organic solvent.
The protonating agents used are chosen to facilitate placing the polyaniline in solution. In particular, there can be used the aliphatic and/or aromatic monoesters and diesters of phosphoric acid, sulfonic acids, and phosphonic acids.
In the case of esters of phosphoric acid, the aliphatic monoesters and diesters are preferred. Preferably, camphosulfonic acid is used as the protonating agent.
The porous substrates used in the invention can be of very diverse materials. For example, insulating polymers, filter papers, glasses, and ceramics may be concerned. The pores of the porous substrates used usually have a mean dimension of 0.2-100 xcexcm.
For the implementation of the method according to the invention, the porous substrate is put in contact with the solution of conductive polymer, either by immersion of the substrate in the solution, or by spraying the solution onto the substrate, for example in the form of an aerosol. After this step, the deposit of polymer is formed within the pores and possibly on the external surface of the substrate, by the simple physical phenomenon of evaporation of the solvent with simultaneous solidification of the conductive phase of the conductive polymer in the form of a uniform layer. Thus, in contrast to the methods heretofore used for introducing a conductive polymer into the pores of an insulating substrate, no secondary product is formed; thus it is not necessary to proceed to the elimination of such products by washing. Moreover, the quantity and the morphology of the deposited conductive layer can easily be controlled by changing the polymer concentration in the deposition solution.
The invention furthermore relates to a solution of polyaniline which can be used for the deposition of conductive polyaniline onto a porous substrate, characterized in that it is constituted by a solution in trifluoroacetic acid of polyaniline in the form of emeraldine base and of a protonating agent.
The protonating agent is advantageously camphosulfonic acid. Preferably, the concentration of polyaniline in the solution is 1-10 g/l.
Other characteristics and advantages of the invention will become more apparent on reading the following description of embodiment examples, which are given, of course by way of example and not limitative, with reference to the accompanying drawings.